Kirsten (Ki)-MSV genome appeared absent from the cell RNA of 5 cloned revertants derived from Ki-MSV-transformed nonproducer BALB/3T3 (Ki-BALB) cells. Except 1 clone, KCR-6113, contained 20 percent of the viral RNA of K-BALB cells. However, MSV was not rescuable from KCR-6113 cells by MuLV, nor inducible by 5-iododeoxyuridine. A new defective transforming virus (MSV-Z) was isolated from mouse S plus L-cells. MSV-Z stock was free of detectable helper virus. The stability, host range and surface properties of MSV-Z differed from known murine C-type oncornaviruses. 5-iododeoxyuridine treatment of S plus L-cells increased MSV-Z titer. Existence of several levels of susceptibility to endogenous murine or feline xenotropic viruses in the homologous cell systems was described. A novel murine oncornavirus with dual eco and xenotropic properties was clonally isolated.